Radiocommunications Agency  EMC Awareness

Railway systems and electric traction

Interference with broadcast reception and PC monitors

U.K. rail infrastructure EMC requirements are unknown

Upgrades to Helsinki trams interfere with hospital on their route

Interference from electric and hybrid road vehicles

Interference with broadcast reception and PC monitors

Description

A report from York EMC Services for the RA has looked at the potential for interference from the various parts of the railway system. The following quotes summarise their conclusions:

“It is well known that the railway electromagnetic environment is much more severe than that found in most commercial and domestic premises. However, in many instances the railway runs very close to such premises. In fact, in the example of an inner city light rail scheme the railway effectively runs along public roads, which brings it into close proximity to non-railway premises and potential victim systems.

There are concerns about radio frequency emissions from railways and their potential to interfere with the operation of commercial radio services and other equipment, such as information technology equipment. Conversely the introduction of new technology radio systems and the potential interference effects from TETRA and 3G communication systems, for example, may impair the safety and reliability of railway equipment….

There is concern amongst CISPR and the radio community that the emission levels and measurement techniques set out in EN 50121 [the railways emissions standard] do not provide adequate protection to radio services.… Some evidence has been found showing that such emissions are capable of interfering with electrical or electronic equipment and radio services operating adjacent to the railway lines… The findings of this study have implications for planned or existing buildings in which IT equipment will be used, where the buildings are situated very close (i.e. less than 10m) to electrified railway lines. There is a significant probability that the passing trains will interfere with PC monitors that are only a few metres away from the lines.”

Commentary

There are two particular EMC threats from railways to the outside world: first, the use in modern trains of high-frequency power inverters for the traction power, which creates high levels of harmonics of the switching frequency. These harmonics have significant content up to and beyond 1MHz, and they are injected directly onto the overhead line, which acts as an efficient long-wave antenna and may be resonant. Long and medium wave broadcast receivers suffer interference from these harmonics, which are usually but not inevitably correlated with the passage of the trains. Filtering of the inverter supply on the train would reduce the threat, but at the expense of cost and weight. Arcing between the current collector and the overhead line or third rail conductor is also a potential source of noise, particularly for the higher voltage systems.

The second problem is that a high DC or AC current flows in the overhead line to supply the traction power. This creates high transient magnetic fields in the neighbourhood of the line, which although they fall off rapidly with distance, can be high enough to upset magnetically-sensitive devices; standard CRT-type VDU monitors are likely to be susceptible, though the flat-panel LCD displays are not. Re-orienting the VDU is a possible solution, but otherwise, offices and other premises using such monitors should not be very close to electrified railways.

References and links

“Potential Electromagnetic Interference to Radio Services From Railways” by T Konefal et al, York EMC Services for the Radiocommunications Agency, RA report ref: AY 4110. Download from http://www.radio.gov.uk/topics/research/topics.htm#emc/

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Filtering
Power harmonic filtering
Ground bonding

UK rail infrastructure EMC requirements are unknown

Description

These quotations are from a hearing that was made public into complaints from the rolling stock suppliers (Adtranz and Alstom) against the infrastructure operator (Railtrack) regarding the inability of Railtrack to provide technical data for acceptance of new rolling stock onto the UK rail network.

Adtranz/Alstom: Railtrack still does not know where its infrastructure is or how it performs. Nor does Railtrack know where its own infrastructure is non compliant with its own norms. The result has been that Railway Group Standards fail to define in key respects, mainly electromagnetic interference and gauging, the actual requirements that Railtrack will demand compliance to when trains are presented for approval.

Railtrack's fundamental failure to know where its infrastructure is, how it performs and the condition that it is in, continues to produce extraordinary turbulence in the requirements for safety acceptance.

We have £500 million vehicles parked in the sidings. All those vehicles are built within existing gauges. They are built with lower interference levels than any of the vehicles in service and we are trying to get those vehicles approved against criteria which are spiralling towards the impossible and left to individuals and subjective appraisal.

Railtrack: Railtrack's inherited infrastructure is 57,000 track circuits of a variety of different types. Many of them have been introduced over a number of years, tens of years, thirty years plus. Many of those track circuits were never designed for the concept of modern traction packages that we currently have being used today.

Most of them were originally designed for something like very statically controlled EMUs etc. A lot of those track circuits are susceptible to certain generated interferences that will come off these new trains. It is an inherent factor of the new train design. The track circuits which were installed and in many cases installed by BR do not necessarily meet today's standards.

Certainly the manufacturing requirement from Westinghouse or Alstom or previous companies that designed these track circuits would have designed it for work at a certain length. For reasons of fitting it to the infrastructure, the infrastructure will sometimes be of varying lengths, sometimes they are much longer in length because clearly if you could just increase it by 50% you can reduce the number of track circuits being fitted to the railway, has a nasty effect of making it far more susceptible to the EMC.

At the time the BR engineers did that, there was perfectly reasonable reason for doing it. They could make the track circuits work, they could make them reliable to operate the railway in a safe manner to detect trains. Unfortunately that same design criteria has made them more susceptible to the design of traction packages today.

Chairman: Sitting where we are if 15 years ago the British Railways Board had mandated that track circuit design ought to be a fairly limited range of track circuits that appeared to be roughly right in terms of emerging traction packages for the next ten years then we might not be sitting here now talking about electro-magnetic interference.

Commentary

The above are quotations taken from the reference below, showing some of the difficulties that train operators were experiencing at that time in trying to meet the EMC requirements of the UK rail infrastructure.

Essentially, it seems that, at the time of this ‘hearing’ Railtrack PLC (the successor to British Rail) were unable to specify the immunity requirements for their ‘track circuits’ (which are safety-critical systems) because they had originally been installed (by British Rail) in a manner that suited the rail network at the time but gave them variable and unknown EMC immunity performance when new technologies were required to be deployed in the rolling stock.

It is exactly this sort of unpredictable interference problem that is preventing the deployment of Eurostar trains north of London. The trains themselves comply with the EMC Directive but it can’t be shown whether they will cause safety problems by interfering with the network they would be required to run on.

References and links

“Hearing RE Adtranz/Alstom complaint about vehicle and route acceptance”, held on Tuesday 9th May 2000 at the Office of the Rail Regulator, London. Document reference 14419 Version 2 - Final. From: http://rail-reg.gov.uk/filestore/docs/adtrans-alstom.pdf, or else go to the Rail Regulator’s home page at http://www.rail-reg.gov.uk and enter 14419 into the search window.

For Eurostar – refer to Electronics Weekly October 23rd 1996, or see item number 41 at http://www.compliance-club.com/archive1/Bananaskins.htm.

Other examples of railway interference problems can be found in the “Banana Skins compendium” via a link from www.compliance-club.com or at: http://www.compliance-club.com/archive1/Bananaskins.htm, especially (at the time of writing) numbers: 12, 42, 94 and 115.

Links to Mitigation techniques

This is an EMC management problem rather than a technical one.

Upgrades to Helsinki trams interfere with hospital on their route

Description

The Helsinki City Transport (HKL) rolling stock is ageing fast. The most recent trams were built 20 years ago. Hitherto, all auxiliary equipment, such as ventilator fan motors were DC and the maintenance of these units was becoming something of a nightmare. Spares were costly and it was a very labour intensive process keeping them in service.

In each HKL tram there were six ventilation fans with DC motors cooling the passenger compartment, brake resistor, and traction motor. The thinking was that one big variable speed (‘inverter’) drive supplying six AC motors was going to be cheaper than several smaller inverters supplying one motor each, so a 15kW unit was mounted in the main electrical panel of one of the trams. The existing cabling was retained because of cost considerations and this connected the various motors in parallel.

EMC problems very quickly surfaced. Not only was the vehicle’s own radio system badly affected, but –crucially – third party electrical equipment also suffered interference, including that of a hospital on the tram’s route.

Commentary

The problem was solved in the end by siting individual inverters, each rated 1.1 - 1.5kW, close to the motors they controlled, to minimise the length of their motor cables.

Typical variable speed drives for AC motors output a pulse-width-modulated (PWM) signal and rely on the inductance of the motor to integrate the sine waveform. The edge-rate (switching speed) of the PWM signals are made very fast to minimise heat loss in the switching devices and improve efficiency. Unfortunately this means that their harmonics extend to very high frequencies, typically more than 1000 times their basic switching frequency. Since a 15kW variable speed AC motor drive would probably switch at around 10kHz, this means their output spectrum could contain significant levels at frequencies up to 10 or 20MHz.

The motor cables in the original trams could easily be as long as 5 metres, which makes them quite effective antennas over a wide frequency range. The shorter the cable the lower its radiating efficiency, but it is also possible to reduce the harmonic emissions by filtering, although this adds cost and weight.

References and links

“Drives on the move” by Les Hunt, Design Products and Applications (dpa) magazine, March 99 (drives supplement, page 29).

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Segregation
Cable routing
Filtering with CM cable-mounted chokes
Cable shielding
Filters

Interference from electric and hybrid road vehicles

Description

In the future, electric traction will probably become the main source of motive power for motor cars and other road vehicles. The electricity may be supplied by rechargeable batteries, fuel cells running on hydrocarbons or hydrogen, an internal combustion engine (the so-called ‘hybrid’ powertrain), or some combination of all three. Overall, these are known as ‘alternative powertrain’ vehicles (the internal combustion engine driving roadwheels via a gearbox being the default powertrain.)

A common aspect of electric traction will be the variable speed electric motor drive that uses pulse-width modulation (PWM). These are already widely used in fixed locations in industry and HVAC (heating ventilating and air conditioning) systems, and are in mobile use in some trains, trams and light railway systems.

The Radiocommunications Agency sponsored two studies, in 2000 and 2002, into the interference issues that might be caused by widespread use of such electric drive technology.

These studies found that there are some areas of concern, where improvements need to be made to the current design and construction practices used in such vehicles, to prevent them from causing an interference problem.

Also, the studies found that existing test methods are not ideal for measuring the true effect of alternative powertrain vehicles on the electromagnetic environment. They propose that new testing methods need to be developed to measure their electromagnetic emissions quickly and meaningfully.

Commentary

The edge-rate (switching speed) of the PWM signals are made very fast to minimise heat loss in the switching devices and improve power conversion efficiency. Unfortunately this means that their harmonic spectrum extends to very high frequencies, typically more than 1000 times their basic switching frequency. A typical switching frequency is 20kHz, and this would mean that the PWM motor drive waveform could contain significant levels of energy at frequencies greater than 20MHz.

The studies measured levels of emissions that would be likely to cause a problem for radio broadcast and communication services at up to 144MHz. At such frequencies the energy in the PWM waveform is relatively small, but on the other hand at such frequencies the cables and structure of the vehicles often make very effective ‘accidental antennas’.

References and links

“A study to assess the possible effects on radio based services of electromagnetic emissions from the proposed increase of electrically powered public and private transport”, by L S Blanchard and D Whitehead of the Transport Research Laboratory, October 2000, unpublished project report PR/SE/186/00, prepared for Radiocommunications Agency. Download from http://www.radio.gov.uk/topics/research/topics.htm#emc

“Investigation of electromagnetic emissions from alternative powertrain road vehicles” by A R Ruddle of the Motor Industries Research Association (MIRA) for the Radiocommunications Agency, 28 May 2002. MIRA report number 01-845060, RA reference AY 4117. Download from http://www.radio.gov.uk/topics/research/topics.htm#emc.

Links to Mitigation Techniques

	Installation	Design & Development	Resources
Design of switch-mode power converters to minimise emissions
Filtering with cable-mounted chokes
Filtering
Shielding of enclosures
Shielding of cables